Fix 'make delete-old-libs' and 'make check-libs' to delete .debug
files created by WITH_DEBUG_FILES. Also cleanup .symbols files from
the period between r244236 when .symbols were supported and r251512
when they were renamed to .debug.

Only propose to delete a .debug file if the corresponding library
itself was deleted already.

Reported by:    des
Reviewed by:    emaste (earlier version)
Approved by:	bapt
MFC after:	3 days